A thermoforming sealing flexible package is a separate sealing package that is formed by two films in the following way: The bottom film is formed into a mold cavity by thermoforming and deep drawing; then an object to be packaged is put into the mold cavity; and finally a second thermal sealing is performed on all sides of the top and the bottom film via evacuation or inflation. The top film (cover film), as a display side of the package, is a multilayer compound film that is transparent or on that is printed graphic and text information of the packaged object; and the bottom film (lower film), as a forming side of the package, is a multilayer compound film suitable for thermoforming and deep drawing, and gets the second thermal sealing.
The thermoforming sealing flexible packaging has currently become a packaging technology widely accepted by the market, being characterized by its high barrier, good sealing, long durability, simple packaging, low cost, wide application scope, etc. However, in view of technology, its functions, properties and characteristics are finally dependent on particularity of the compound packaging material from the multilayer compound top and bottom film.
In the prior art, the multilayer coextrusion compound film is usually compoundly formed from multilayer materials with different functions through a coextrusion process. Theoretically, it possesses great flexibility on both function and structure, and the product can be flexibly designed and arranged according to different requirements on properties. For example, such materials as PA, EVOH and PVDC can be adopted to realize such functions as oxygen barrier and aroma conservation; such materials as EVA, PE, PP and PVDC can be adopted to realize such functions as water barrier; EVA, PE and MPE resin can be adopted to realize the thermal sealing function; and an adhesive resin can be adopted to realize an adhesive function. However, it is not easy to practically develop a flexible packaging material with an excellent cost performance. The main reasons are as below:
First, there is a conflict between property and price in material selection; that is, a material with excellent properties such as in barrier, deep drawing and strength usually has a higher price, and it is very difficult to obtain a product that is inexpensive and high in quality. For example, PA (nylon) is a high quality engineering plastic, which exhibits better properties in thermoforming, deep drawing, strength, etc., and can meet requirements of most occasions; however, it is expensive, being almost twice as much as a universal plastic PP (polypropylene).
Second, there is a conflict between material and process performance in the functional design; that is, though some materials possess excellent properties and can meet functional design requirements, there are a series of problems in processing. For example, when a material is formed by coextrusion, the material possessing excellent properties and functions such as in barrier, deep drawing, skin packaging, antipiercing and anti-pollution sealing against the product, it is usually subject to restriction in material selection for structure due to difference in processing conditions. For example, being a copolymer of vinylidene chloride and chloroethylene, PVDC (polyvinylidene chloride) possesses high crystallizability and very good combination barrier properties, and is a packaging material with high barrier and good tenacity as well as low-temperature thermal sealing, thermal contractility and chemical stability. Its biggest characteristic is that, without a hydrophilic group in the polymer, it possesses a good water-vapor barrier property, while its gas barrier property is independent on the environmental humidity. However, its weaknesses are that it is so soft that it is poor at operational property; it is strong in crystallizability and easy to be cracked and perforated, and has poor aging resistance; and particularly, being a typical thermally sensitive resin, it is very sensitive to temperature during the forming process, and highly inclined to decompose to produce chlorine gas and hydrogen chloride. Therefore, it will be very difficult to compoundly form at a time the thermally sensitive resin polyvinylidene chloride with other high-temperature molten resin materials (e.g. the temperature of nylon resin melt can be up to 300° C.). All the current merchant BOPA/PVDC/PE compound film is compounded from a BOPA film and a PVDC film, which are required to be first produced, respectively. Comparatively, though possessing better processing and barrier property, EVOH (ethylene-vinyl alcohol copolymer) has a biggest weakness that its barrier property decreases dramatically in a high humidity environment, i.e. it is good at oxygen barrier but poor at humidity barrier, and its oxygen barrier property decreases dramatically with temperature.
Third, there is a conflict between structure and process in the product design; that is, in the multilayer coextrusion compound film design, an asymmetrical structure, compared with a symmetrical structure, obviously possesses the advantage of considering the follow-up processing characteristic. However, it is because of this that the most outstanding problem is that the film is inclined to be curled during the coextrusion forming process. The mechanism is as below: During the coextrusion forming process of the asymmetrical structure formed by a crystalline resin and a noncrystalline resin under the same temperature and pressure, an unbalanced stress will surely be produced due to difference in crystalline temperature and velocity of the materials, thus directly curling the film. For example, when a product with a seven-layer asymmetrical structure is produced, the outer nylon layer and the inner polyethylene layer are different in crystalline temperature and velocity during the cooling process of molten coextrusion; that is, the crystalline temperature of nylon is 180° C.˜140° C. while the crystalline temperature of polyethylene 120° C.˜80° C.; therefore, nylon is first crystallized and shrinks by 2˜3% to be set into solid, while polyethylene is then crystallized and solidified and shrinks by 2˜3%, and finally the compound film is curled under action of the stress. This weakness not only makes winding of the film difficult, but also directly affects a future second processing of the compound film. For example, it is always difficult to produce bags and thermoform with the curled film.
The following conclusion can be drawn based on all mentioned above: For the flexible packaging film material from multilayer coextrusion compound forming, it is very difficult to design and develop a flexible packaging material with an excellent cost performance for a kind of packaged object.